For the high efficiency collection of fly ash from coal-fired boilers, electrostatic precipitators and fabric filters are almost exclusively used. In this application they generally operate at temperatures greater than 250.degree. F., with 300.degree. F. being typical. Although lower temperature operation would be advantageous for both energy efficiency and equipment size, this is normally not possible because of the sulfuric acid dewpoint; if flue gas is cooled below the acid dewpoint, an acid mist forms which subsequently attacks the downstream equipment. In coal fired boiler applications, the acid dewpoint is determined by the concentration of sulfur trioxide in the flue gas, which may be in the range of 5 to 30 ppm.
It is known to treat flue gases with an alkali by injecting, for example, calcium compounds into the flue gas or into the boiler and examples of patents disclosing such are:
______________________________________ 3,481,289 Oda et al 3,520,649 Tomany et al. 4,185,080 Rechmeier 4,309,393 Nguyen ______________________________________
It is also known to treat flue gases by lowering the gas temperature via water injection or boiler preheaters or both and examples of patented art relevant thereto are:
______________________________________ 1,329,737 Wolcott 1,329,818 Wolcott 3,523,407 Humbert 4,208,383 Kisters et al. ______________________________________